Background/Objectives: The association between gastroesophageal reflux disease (GERD) and chronic rhinosinusitis (CRS) has been debated, with conflicting evidence regarding its magnitude and clinical significance. This systematic review and meta-analysis aimed to evaluate the epidemiological association between GERD and CRS, assess the potential pathophysiological mechanisms, and determine the impact of diagnostic methodology on the observed relationship. Methods: A literature search was conducted across PubMed, Embase, the Cochrane Library, and Web of Science for observational studies published up to January 2026. Studies reporting the prevalence or odds of GERD in adult patients with CRS compared with controls were included. Methodological quality was assessed using ROBINS-E, JBI, and Hoy et al. Data were pooled using random-effects models with Hartung–Knapp–Sidik–Jonkman adjustment. Heterogeneity was explored using subgroup and meta-regression analyses. The certainty of the evidence was graded using the GRADE approach. Results: Twenty-two studies, involving 45,618 participants, met the inclusion criteria. Patients with CRS had significantly higher odds of having GERD than controls (OR 5.39; 95% CI: 2.06–14.11; p < 0.001). This association was corroborated by longitudinal data (aOR, 1.77; 95% CI: 1.44–2.17; p < 0.0001) and genetic evidence from Mendelian randomization. Subgroup analysis revealed that the magnitude of association was significantly stronger in studies utilizing objective diagnostic tools, such as pepsin assays (OR 15.20) and pH monitoring (OR 6.94), compared to subjective questionnaires (OR 1.43). This association was significant for CRS with nasal polyps (CRSwNP) and CRS without nasal polyps (CRSsNP). Meta-regression analysis indicated a stronger association in younger populations (p = 0.034). Moderate-certainty evidence supports a positive epidemiological association between GERD and CRS, although substantial between-study heterogeneity (I² = 69–89%) and wide 95% prediction intervals limit the precision and generalizability of the pooled estimates. Conclusions: An association between GERD and CRS is observed in studies using objective markers of extraesophageal reflux, whereas the association is attenuated and not statistically significant when reflux is assessed by subjective questionnaires. Objective reflux assessment may be considered in selected patients with medically and surgically refractory CRS, but routine reflux testing in unselected CRS patients is not yet supported by the available evidence.